The electrical steel sheets are soft magnetic materials widely used as iron cores for transformers, motors or the like. Among them, the grain-oriented electrical steel sheets are excellent in the magnetic properties because their crystal orientations are highly accumulated into {110}<001> orientation called as Goss orientation, so that they are mainly used as iron cores for large-size transformers or the like. In order to decrease no-load loss (energy loss) in the transformer, the iron loss is required to be low.
As a method for decreasing the iron loss in the grain-oriented electrical steel sheet, it is known that the increase of Si content, the decrease of sheet thickness, the high accumulation of crystal orientations, the application of tension to steel sheet, the smoothening of steel sheet surface, the refining of secondary recrystallized grains and so on are effective.
As a technique for refining secondary recrystallized grains among these methods is proposed a method wherein the steel sheet is subjected to a heat treatment by rapid heating in decarburization annealing or rapid heating just before decarburization annealing to improve primary recrystallized texture. For example, Patent Document 1 discloses a technique of obtaining a grain-oriented electrical steel sheet with a low iron loss wherein a cold rolled steel sheet with a final thickness is rapidly heated to a temperature of not lower than 700° C. at a rate of not less than 100° C./s in a non-oxidizing atmosphere having PH2O/PH2 of not more than 0.2 during decarburization annealing. Also, Patent Document 2 discloses a technique wherein a grain-oriented electrical steel sheet with a low iron loss is obtained by rapidly heating a steel sheet to 800-950° C. at a heating rate of not less than 100° C./s while an oxygen concentration in the atmosphere is set to not more than 500 ppm and subsequently holding the steel sheet at a temperature of 775-840° C. which is lower than the temperature after the rapid heating and further holding the steel sheet at a temperature of 815-875° C. Further, Patent Document 3 discloses a technique wherein an electrical steel sheet having excellent coating properties and magnetic properties is obtained by heating a steel sheet to not lower than 800° C. in a temperature range of not lower than 600° C. at a heating rate of not less than 95° C./s with properly controlling an atmosphere in this temperature range. In addition, Patent Document 4 discloses a technique wherein a grain-oriented electrical steel sheet with a low iron loss is obtained by limiting N content as AlN precipitates in the hot rolled steel sheet to not more than 25 ppm and heating to not lower than 700° C. at a heating rate of not less than 80° C./s during decarburization annealing.
In these techniques of improving the primary recrystallized texture by rapid heating, the temperature range for rapid heating is set to a range of from room temperature to not lower than 700° C., whereby the heating rate is defined unambiguously. Such a technical idea is attempted to improve the primary recrystallized texture by raising the temperature close to a recrystallization temperature in a short time to suppress development of γ-fiber (<111>//ND orientation), which is preferentially formed at a common heating rate, and to promote the generation of {110}<001> texture as a nucleus for secondary recrystallization. By applying these techniques are refined crystal grains after the secondary recrystallization (grains of Goss orientation) to improve the iron loss property.